In the field of dental treatment such that a local area to obtain CT image, namely an imaging interested area, is only a part of a relatively small area like a human head, for example one or two teeth, an X-ray CT apparatus in which an X-ray rotary center is fixed on the imaging interested area, X-ray is radiated in a circulating manner only on the imaging interested area and the image of the X-ray absorption coefficient of the imaging interested area has been heavily used recently because its X-ray dosing amount on a patient is remarkably reduced comparing to a prior art and the apparatus is small sized because of vertical rotary center.
In this filed, there has been a large demand of a curved plane sectional image which is a transmitted X-ray image of a curved sectional area of an entire dental arch and an X-ray CT apparatus capable of obtaining a curved plane X-ray sectional image has been provided. The curved plane X-ray sectional image of the curved sectional area of an entire dental arch is called as a dental X-ray panoramic image.
For example, in the X-ray CT apparatus disclosed in JP-A-10-225455 by the present applicant, a curved plane X-ray tomography is executed by moving an X-ray rotary center during X-ray radiation and an X-ray CT is executed when the X-ray rotary center is fixed on the area of an object to obtain a CT image, namely an imaging interested area. However, the X-ray rotary center is designed to be movable for the curved plane X-ray tomography, so that mechanical accuracy required for fixing the rotary center during X-ray CT is not strictly held because of backlash of a gear. Therefore, the adverse effect on the accuracy of CT image caused by deflection of the X-ray rotary center has not been ignored, thereby causing the affect on X-ray CT by using both as the curved plane X-ray tomography apparatus and as the X-ray CT apparatus.
The X-ray CT apparatus disclosed in JP-A-2000-139902 is proposed by the applicant of the present invention, in case of curved plane X-ray tomography, the X-ray rotary center is fixed on the center of a virtual local region which includes all the orbit of X-ray required for obtaining prior curved plane X-ray sectional image, conical X-ray beam is radiated on the virtual local region, and thus obtained X-ray transmitted images are patched together to obtain the curved plane X-ray sectional image similar to the prior art.
However, in this apparatus, slit control for radiating X-ray conical beam is complicated and the distance between the X-ray rotary center and the tooth is different depending on the position in the dental arch, thereby requiring to arrange the pace of expansion to obtain the same size images as a whole.
Further in this apparatus, when an X-ray transmitted image is backprojected, a three-dimensional X-ray absorption coefficient of the dental arch is calculated and the curved plane X-ray sectional image can be obtained by reconstructing the coefficient, however, the length of process time has been problem.
Therefore, the X-ray CT is a main object of the invention, thereby causing adverse effect to the curved plane X-ray tomography.
Any one of the above-mentioned X-ray CT apparatus is capable of curved plane X-ray tomography in addition to X-ray CT, however, it cannot execute both a curved plane X-ray tomography and an X-ray CT as well as an exclusive apparatus for curved plane X-ray tomography or X-ray CT, or more preferable than it. Further, the apparatus cannot achieve smooth link of both tomography such that a local X-ray CT is executed utilizing the result of curved plane X-ray tomography while an object is fixed to the apparatus.
In JP-B-2-29329 proposed by the applicant of the present invention, an X-ray imaging apparatus has been disclosed such that a panoramic X-ray imaging for a dental arch is executed with an X-ray image sensor comprised of an X-ray multiplier. In this apparatus, a panoramic radiography on the curved plane along the dental arch has been possible, however, X-ray CT has not been possible.